Gravity, Friction and Simple Machines

BY: BRIANNA SHIELDS

Do Now

• 1. How do you calculate total force if the two forces are in opposite directions?

• 2. Which of Newton’s laws of motion explains inertia?

• 3. What happens to an object if two forces are equal but in opposite directions?

GOAL

• To relate gravity and weight

• To define friction and air resistance and design ways to increase it or decrease it

• To define work

• To describe various simple machines

• Gravity • 1. Force of attraction-pulls things downward

• 2. Objects fall at a rate= 9.8 m/s2

– acceleration due to gravity

Example

1 sec

2 sec

3 sec

4 sec

5 sec

6 sec

7 sec

After falling for 7 seconds what is the acceleration of the ball?

Speed of a falling object

V = 9.8 x t

(V= a(g) x t )

Free Fall

• Terminal Velocity

• Object no longer accelerates- it travels at a constant speed

• Law of Universal Gravitation

• 1. All objects have gravity• 2. Massive objects = greater

gravity• 3. Objects close together= more

gravitational attraction

Less Gravity More Gravity

Less Gravity More Gravity

• Weight • 1. Amount of gravity that pulls on an object– Closer to core = more gravitational pull

• 2. Weight changes with location

• WEBSITE: Your weight on other worlds

• Weight • 3. W= m x 9.8– Weight= mass x acceleration due to

gravity

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Weight Calculations

• What would a 10kg book weigh in newtons?

• W = ma = 10kg x 9.8 m/s^2 = ?

• What would a 50kg person weigh in newtons?

• W = ma= 50kg x 9.8 m/s^’2 = ?

If a feather and a hammer are dropped at the same time, which will hit the ground first?

QuickTime™ and aTIFF (Uncompressed) decompressor

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QuickTime™ and aTIFF (Uncompressed) decompressor

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Why does the hammer hit the ground before the feather?

• Air resistance

• Prevents objects from falling at 9.8 m/s

• Slows objects down• Object rubs against air molecules• Pushes upward on objects• Due to object’s shapeQuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

If a feather and a hammer are dropped at the same time on the moon, which will hit the ground first?

QuickTime™ and aTIFF (Uncompressed) decompressor

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QuickTime™ and aTIFF (Uncompressed) decompressor

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Find out by watching “Brain Bytes” NASA Vodcast

• Friction • 1. Exists Where 2 surfaces touch• 2. Acts in direction opposite to

object’s motion

Friction acting on an incline

• Often, picking up a heavy object is actually easier than trying to slide it across the floor

FRICTION

Pushing

• Types of friction

• 1. Sliding- produced when solid objects slide over each other

• 2. Rolling Friction- produced by wheels

• 3. Fluid friction- created when an object moves through/across a fluid (water, oil or air)

Examples of Friction

• Come up with atleast two examples of each friction: sliding, rolling and fluid

• Come up with atleast two examples during which friction would be helpful

Label with the type of friction:

Label with the type of friction:

Label with the type of friction:

• The movement of lubricated engine parts

Label with the type of friction:

Label with the type of friction:

Label with the type of friction:

Label with the type of friction:

When Friction is Helpful

Compare these two animations. How can the differences be explained?

No Gravity: What will happen when the banana is shot from

the cannon?

With Gravity: What will happen when the banana is shot from

the cannon?

You will view a photograph of two balls being released from an

apparatus. Which will hit the ground first? The one dropped straight down or the one shot

out sideways?Website: http://www.

fearofphysics.com/XYIndep/xyindep.html

Projectile Motion Tutorial Online: http://www.mhhe.

com/physsci/physical/giambattista/proj/projectile

.html

• Work • A force acting through a distance to move an object– Must be moved over a distance

– Object must move in direction of force

• Work = force x distance

• Work • Examples– Pushing a desk

– Throwing a baseball

– Lifting a shovel

• Not Examples– Carrying a grocery bag

– Pushing on a wall

– Carrying a shovelful of snow

Work or Not??? Explain

• Picking up a grocery bag• Carrying a grocery bag to the door• Throwing a baseball 20 meters• Lifting a suitcase to put into the

overhead luggage compartment• Pushing against a cement slab until

exhausted

Work or Not??? Explain

• Carrying a bag of cat litter home from the store

• Slamming a tennis ball across the net

• Studying all night for the science test

• Finger pushing down the RETURN key on a computer

Work or Not??? Explain

• Carrying a shovel full of snow from the driveway to the lawn

• You and a friend pushing a heavy piano across a wooden floor

• Standing for half and hour in the freezing cold waiting for the bus to come

Work Example

• If you lifted an object weighing 200 N through a distance of 0.5 m, how much work would you do?

• W = F x D = 200N x 0.5m = 100nm

Work Example

• If you picked up a rock weighing 350 N for a distance of 1.0m, how much work would you do?

• W = F x D = 350N x 1.0m = 210 nm

• What do machines do?

• Change small forces into large forces

• Allow small forces to move large objects

• Change the size and direction of the force

• Doesn’t reduce work- divides it into smaller jobs

• Inclined plane

• Ramp, slanted surface• Ex: truck ramp

• Wedge • Moving inclined plane with edges

• Ex: knife, scissors, key

• Screw • Bar wrapped with threads for fastening

• Ex:– Lightbulb– Jar Lid

• Lever • Bar that pivots up and down• Ex: hammer, teeter toter

• Pulley • Rope wrapped around a wheel• Ex: window shade pull

• Wheel and axle

• 2 different sized spinning circles• Ex: fan, sawblade, screwdriver

Machine Assessment

• Work with your group members to create your own examples (atleast two) for each machine:– Pulley– Lever– Wedge– Screw– Inclined plane– Wheel and axle

Simple Machine Websites

• Site 1: Edheads - Simple Machines Activities - Lever ...

• Site 2: Simple Machines Made Simpler - by The Spice B...

Homework

• Write the following assignment into your agenda:– Physics Homework